From providing a window on fundamental symmetries to probing the strong interaction, LEAP’03 covered the many parts played by low-energy antiprotons from accelerators, as John Eades reports.
LEAP’03, the latest in the series of biennial low-energy antiproton-physics conferences, could not fail to be topical this year. Running from 3-7 March, it began with the latest news on the production of antihydrogen atoms at energies low enough to permit them to be studied by laser spectroscopy, and ended with progress reports on two new antiproton facilities (support for one of which had been announced by the German government just days before).
This somewhat unusual time of the year for LEAP – previous conferences in the series have always taken place in the Autumn of even years – is attributable to the frenzied activity at CERN’s Antiproton Decelerator (AD), which now keeps many likely experimental participants busy between May and October. This year the meeting moved to Yokahama, where its packed programme more than compensated for the uncharacteristically wet and windy March weather. Some 60 talks reflected the recent surge of activity in what has become an exceedingly dynamic field.
An accelerator-produced antiproton normally spends only a brief instant in the world of matter. However, in this short time it can play many parts, from probing fundamental symmetry principles, to the study of atomic collisions, atomic bound states and nuclear physics. The initial session on discrete symmetries and antihydrogen was devoted to the current status of CPT invariance, the testing of which is perhaps the most powerful driving force behind current experimental activity in this field. Many particle physicists accept CPT invariance almost as an article of faith. They forget that, like Euclidean geometry, it is indeed a theorem, based on cherished but not entirely indispensable axioms such as Lorentz invariance (LIV), a feature the gravitational field only possesses locally. In his theoretical review, Nick Mavromatos of King’s College, London, concentrated on the fact that gravitation has shown itself to be particularly resistant to quantization under the terms of the CPT theorem. Pointing out that it is not difficult to construct models containing parameters that violate LIV and other CPT axioms, he neatly connected ultra-high, Planck-mass scale energies with ultra-low ones, by suggesting that experiments on neutral mesons, slow neutrons and in particular antihydrogen atoms, can place bounds on these parameters. In a backward look at data from the KTeV, NA48 and CPLEAR collaborations, Yoshiro Takeuchi of Nihon University, Tokyo, analysed these in terms of CP- and T-violation parameters and limits on CPT violation, concluding that relative to the level of K0-K0bar mixing, CPT violation is currently constrained in the meson sector to a few parts in 105 at best.
A starring role
The spotlight then turned on the experimentalists and the antiproton’s most recent starring role in the synthesis of large numbers of antihydrogen atoms. Nathaniel Bowden and Joseph Tan of Harvard brought participants up to date on the latest developments in antihydrogen synthesis from the ATRAP experiment. They reported on ATRAP’s use of positrons in a nested Penning trap to cool antiprotons to the cryogenic energies necessary for the recombination reaction between the different particles to take place, and on the field ionization method used for detection. The latter makes it possible to observe antihydrogen atoms under background-free conditions and to measure, for the first time, the distribution of principal quantum numbers for the synthesized atomic states. Makoto Fujiwara of Tokyo and Germano Bonomi of CERN described the production, detection and temperature dependence of antihydrogen atoms in the ATHENA experiment, which uses a similar Penning trap but with a distinctive open and modular design. This allows, among other things, a buffer gas to be introduced on the positron side, in which continuously introduced positrons from 22Na dissipate enough energy to prevent them re-emerging from the trap. Differential pumping then maintains a good-enough vacuum to ensure the survival of antiprotons for many hours on their side of the trap. ATHENA identifies antihydrogen events without ambiguity by detecting the simultaneous annihilation of their component positrons and antiprotons. New techniques for probing the positron plasma that rule out alternative but unlikely interpretations of the data have recently been introduced.
Window on the world
Antiproton beams have long provided a window on the shadowy world of glueballs, hybrids and quarkonia. In his review of this rich source of information on hadron physics, Ted Barnes from Oak Ridge looked both backward to LEAR and forward to future antiproton machines. Surviving glueball candidates from the era of LEAR, which ended in 1996, include the f0(1500) and, with less confidence, the f0(1710), while exotics include the π1(1400) and π1(1600). The advent of new antiproton sources at GSI and the Japan Proton Accelerator Research Complex (J-PARC; previously the Japan Hadron facility) now promises to open this window once again. Several more specific talks reviewed topics such as charmonium states from proton-antiproton annihilations in the Fermilab experiment E835, and the future Proton Antiproton Detector Array (PANDA) at GSI.
Low-energy antiproton beams can readily be stopped in matter targets. Before fully coming to rest, the antiprotons eject electrons from nearby target atoms and remain bound in their place. Once installed in this antiprotonic atom, they undergo complex cascades through electromagnetism-dominated states before coming within the range of strong interactions. However, it is only in antiprotonic helium that this cascade is known to last more than a few picoseconds. In this case, the microsecond-scale annihilation lifetimes of some atomic states fortuitously makes them accessible to laser spectroscopy, thus ensuring that antiprotonic helium can, in some respects, rival antihydrogen as a benchmark for studies of CPT invariance.
Attention on the second day, therefore, turned once again to the AD, with a session on the experimental and theoretical studies of antiprotonic helium. Masaki Hori of CERN reported on the limit on the antiproton charge and mass that can now be deduced from measurements of transition frequencies in the antiprotonic atom to a few parts in 107. The new limit results in part from the recent addition of a decelerating radio-frequency quadrupole (RFQD) to the ASACUSA beamline. This reduces the beam’s kinetic energy from the MeV to the keV scale, and so allows the antiprotons to be stopped in very-low-density helium, with concomitantly smaller systematic corrections to the measured frequencies. Further impetus, expected from two-photon laser techniques and more advanced laser systems, may soon improve the precision of the frequency measurement to several parts in 109, and so also permit spectroscopy of the two-body antiprotonic helium ion (pbar He++). Jun Sakaguchi of Tokyo described the ASACUSA microwave-spectroscopy experiment on antiprotonic helium, which has allowed the antiproton orbital magnetic moment to be determined from the hyperfine splitting of atomic levels to a few parts in 105. The QED calculations that make all the above interpretations possible, were described by Vladimir Korobov of JINR Dubna, and further talks dealt with the physical chemistry of the antiprotonic helium atom.
Antihydrogen experiments dominated again in the following session on the future programme for the AD. Cody Storry of Harvard introduced a novel antihydrogen production mechanism that is being considered by ATRAP. A beam of caesium atoms previously excited into Rydberg states by a laser beam passes through the positron cloud confined in a Penning trap, where they produce positronium atoms that are also in Rydberg states. These have a much higher recombination cross-section with trapped antiprotons than is the case for ground-state positronium. The next major goal for both ATHENA and ATRAP is to begin laser spectroscopy of cold antihydrogen atoms, and several different schemes, including laser-stimulated recombination and ionization, are being investigated. From ASACUSA, Yasunori Yamazaki of RIKEN and Tokyo presented the idea that positrons and antiprotons may be confined in the same region of a trap incorporating a magnetic field “cusp”, while Eberhard Widmann, also from Tokyo, showed that a precision measurement of the ground-state hyperfine splitting in antihydrogen must now be seriously considered to fall within the AD’s “line of sight”. Throughout the history of modern physics, experiments with atomic beams have proved extremely fruitful in studying the hydrogen atom with high precision, and these latter two topics opened up the idea that the same can be true for antihydrogen.
The research arena then moved from earthbound laboratories to the atmosphere and space, where the search for cosmic antimatter has been under way for many years. Catherine Leluc of Geneva reviewed the status of the ALPHA Magnetic Spectrometer (AMS02). This is due for launch to the International Space Station in October 2005, but a pilot version (AMS01) has already been flown on the shuttle mission STS-91 in 1998. The AMS02 detector now incorporates improved acceptance and redundancy into its search for antimatter and dark matter in cosmic rays. The third-generation high-altitude balloon experiment BESS-Polar was discussed by Mitsuaki Nozaki of Kobe. This will be used to study low-energy cosmic-ray antiprotons in detail in a superconducting magnetic spectrometer, and is expected to have a 10-20 day flight through the top of the polar atmosphere in 2004. Finally, Piero Spillantini of Firenze described PAMELA, a successor to several balloon-borne experiments, which will be launched later this year into quasi-polar orbit on the Russian Resurs-DK1 satellite from the Baikonur Cosmodrome.
Before they can be captured into atomic states, antiprotons produced at accelerators must lose some nine orders of magnitude of kinetic energy, and as is the case for other particles, their interaction with matter over this range is of crucial importance. The fact that antiproton projectiles are both heavy and negatively charged has far-reaching consequences for their behaviour when passing through matter. Capture/loss processes and the excitation of target electrons drastically modify the Bethe-Bloch formula at the velocity scale of the electron orbitals in the target material. The RFQD installed in ASACUSA’s beamline has shed new light on this experimentally dark area, and Ulrik Uggerhoj from Aarhus was able to report on the latest results on stopping-power measurements made with antiproton beams from 1 to 100 keV in C, Al, Ni, Au and LiF foils. The results of theoretical approaches to the understanding of collisions of antiprotons with hydrogen and helium atoms, ions and molecules, and to the explanation of ionization phenomena in the low-velocity domain, were presented by John Reading of Texas A&M and several other speakers.
The final curtain
The life of an antiproton ends when it comes within range of the strong interaction, either after an atomic cascade or (occasionally) by a direct in-flight hit on the nucleus. It is then that it plays its final research role, as a nuclear probe. The AD as presently constituted is not easily adaptable for such studies, but with GSI and J-PARC now on the horizon, Josef Pochodzalla of Mainz was able to look forward to using antiprotons from these machines for the large-scale production of single and double Λ-hypernuclei. The weak decays and gamma-ray spectra of these hypernuclei can elucidate hyperon-hyperon and hyperon-nucleon interactions, measure fundamental properties of the hyperons themselves, and produce genuine hypernuclear states with symmetry properties unavailable to ordinary nuclei. These antiprotons that have suffered atomic capture can eventually de-excite to atomic ground states that “graze” the nucleus, so their annihilation constitutes an effective probe of the nuclear surface. This aspect occasioned both backward and forward glances in reports on new analyses of data from the PS209 experiment at LEAR and the possibility of similar studies at ASACUSA.
The final day of the LEAP’03 conference was appropriately devoted to the current and future antiproton facilities. The morning session opened with a review by Tommy Eriksson from CERN of the present status of and future prospects for the AD machine, where the name of the game is “ever lower energies”. The AD is now operating close to its design specifications, with pulses containing 107 antiprotons being reliably delivered at an energy of 5.3 MeV every 100 seconds. Research at even lower (keV) beam energies has now been strongly boosted by the RFQD, which has permitted several million antiprotons to be captured in the Tokyo Penning trap and cooled to cryogenic energies. Naofumi Kuroda of Tokyo discussed their extraction in the form of a beam of antiprotons with kinetic energies on the eV scale. A new feature of the AD programme, which was described by Carl Maggiore of Pbar Medical, is an investigation with a 300 MeV/c (25 MeV) beam of a possible therapeutic role for antiprotons. This beam, astronomically high in energy for most other physicists, will soon be used to investigate the relative biological effect of antiprotons on biological cell samples.
In the final session on the topic of future antiproton facilities, Walter Henning, GSI’s director, described the laboratory’s new project and its potential for antiproton physics. The large-scale expansion of the GSI-Darmstadt laboratory, the funding of which has only very recently been agreed by the German federal government, will be carried out in two stages, with a 25% external contribution. One of its key elements will be the provision of antiproton beams below 15 GeV. Shoji Nagamiya, director of the J-PARC project, outlined progress on this new Japanese accelerator complex, centred on a 50 GeV proton synchrotron at Tokai, 150 km north-east of Tokyo. Planning has been under way since 2001 and is now rapidly gathering momentum. With financing amounting to some ¥134 billion (€980 million), Phase 1 is expected to produce its first beams in 2008. An opening ceremony was held in October 2002, and 30 letters of intent had been received by the end of December 2002, one-third each from Japan, Europe and North America. Both GSI and J-PARC now actively encourage the voice of antiproton users to be heard in the planning of their experimental programmes.
The best yet
The smooth organization of LEAP’03 largely resulted from the efforts of Eberhard Widmann and Ryugo Hayano of Tokyo University, with financial assistance being provided by RIKEN, KEK and the Antimatter Science Project at the University of Tokyo. Viewing the busy and spectacular Yokohama bay through the Sangyo Boeki Centre’s windows during the coffee breaks, the 100 participants could all agree that the form and content of the conference programme was the best yet.
